Most of the current crew breathing masks are equipped with oxygen regulators using pneumatic technology to satisfy this requirement. In this technology, ambient air is sucked through a dilution gas supply line by a Venturi which provides suction by high velocity flow of breathable gas. An aneroid capsule (called also altimeter capsule) regulates the altimetric oxygen enrichment by adjusting the section of the dilution gas supply line. Such demand regulators are known from the documents U.S. Pat. No. 6,994,086, FR 1 484 691 or U.S. Pat. No. 6,796,306. As the oxygen enrichment depends on the section of the dilution gas supply line controlled by the aneroid capsule clearance, the oxygen consumption cannot be optimal for all of the cabin altitude range and/or for all of the breathing ventilation.
The need to save oxygen has lead to the development of electropneumatic regulator as described in the documents U.S. Pat. No. 4,336,590, U.S. Pat. No. 6,789,539, US 2007/0107729 or US2009/0277449. The demand regulators disclosed in these documents comprise an electrical valve controlled by an electronic circuit for adjusting the rate of oxygen in the respiratory gas. These demand regulators electrically control both the pressure of the respiratory gas relative to the cabin pressure and the oxygen rate of the respiratory gas. Reliability of these demand regulators is linked to the reliability of the electronic circuit or the electrical power supply. For example, in case of electrical power supply breakdown, these demand regulators do not protect the user against hypoxia or fire smoke.
Some improvements have been made in the past by adding a pneumatic demand regulator to the electro-mechanical regulator, the pneumatic demand regulator providing a backup solution which is used only in case of electrical failure. But this leads to systems far more complex and bulky than the classical regulator with Venturi and aneroid capsule for dilution control.
So, it is already known, for example from a first embodiment disclosed in document U.S. Pat. No. 6,789,539, a demand regulator for aircraft breathing device comprising:                a respiratory chamber supplied with respiratory gas comprising breathable gas and dilution gas,        a breathable gas supply line to be connected to a source of breathable gas and supplying the respiratory chamber with breathable gas,        a dilution gas supply line to be connected to a source of dilution gas and supplying the respiratory chamber with dilution gas,        a first adjusting device adjusting the pressure in the respiratory chamber, and        a second adjusting device adjusting the rate of dilution gas in the respiratory gas supplied to the respiratory chamber, the second adjusting device comprising a dilution valve disposed in the dilution gas supply line and the dilution valve being movable between a retracted position and a protruded position.        
This demand regulator appears satisfying in normal condition, but does not protect the user in case of electrical failure. The aim of the invention is to improve the reliability of this demand regulator.
Document U.S. Pat. No. 6,789,539 further discloses a second embodiment of demand regulator, wherein the first adjusting device is of non-electrical type, the demand regulator further comprises a third adjusting device controlling the flow rate of breathable gas in the upstream portion of the breathable gas supply line and the second adjusting device comprises an altimeter capsule. Such a demand regulator could be quite satisfying in case of electrical failure. But, it is complicated and above all it is very difficult to settle in normal conditions because the supply of breathable gas is controlled by both first adjusting device and second adjusting device.